This new page is intended for material on the role of disease in evolution, including
the ancient innovation of sex, the more recent one of sleep, the origin of allergies
(see below), and the role of disease in the history of civilization. In evolutionary
biology, the term "parasite" is commonly used to denote bacteria, viruses, and
other small organisms making a living on us.

Genetic information flow in a biological cell can be described as "transcription"
to "translation". In general, transcription is the process whereby genome DNA
code is transcribed into RNA code, ultimately in the form of "messenger RNA" (mRNA),
and messenger RNA in turn is essentially a tape that feeds into ribosomes one
triplet nucleotide base unit (codon) at a time to program the sequence of amino
acids of a polypeptide, the protein synthesis process called "translation": the
messenger RNA polymer tape goes into the ribosome codon by codon, and the synthesized
protein polymer product comes out of the ribosome amino acid by amino acid.

In the context of viruses, what is important is that viruses do not contain the
protein catalysts (enzymes) required for their own replication, and they must
therefore parasitize host cells and by one of various means get the ribosomes
of such host cells to manufacture viral proteins, including the enzymes necessary
to catalyze the replication of the viral genome. The final complete virus (virion)
is thus assembled from components provided by the host cell, the virus essentially
programming the synthesis of its own parts.

Concerning the genomes of viruses, there are two general types, the DNA viral
genome and the RNA viral genome. Only in certain viruses do we find genomes consisting
of RNA; all biological cells (and they are all potential hosts for viruses) contain
DNA genomes. There are more than 2500 groups of different viruses now recognized
and at least partially characterized, and a variety of logical classifications
of viruses exist, but certainly one useful broad classification scheme differentiates
viruses into various DNA viruses and various RNA viruses, with each type of virus
having a more or less different challenge once it enters a host cell. But in each
case, for both DNA and RNA viruses, the general challenge is the same: directly
or indirectly the viral genome must bring about the production of the messenger
RNAs necessary to get the host ribosomes to produce the proteins necessary for
viral replication.

In general, with some types of RNA viruses, the RNA genome ("plus-sense"; "positive-strand")
can itself act as messenger RNA for host ribosomes; while other types of RNA viruses,
the RNA genome ("minus-sense"; "negative-strand") must first produce
a complementary RNA, which then acts as messenger RNA for the host ribosomes.
The replication process in minus-sense RNA viruses is complex, since host cells
do not carry enzymes that can polymerize complementary RNA from an RNA template,
and such viruses therefore must carry their own special enzymes ("RNA-dependent
transcriptases") to achieve this synthesis.

The human immune system may be calibrated to a certain parasitic load. If the
load is unusually (abnormally) light, the immune system may become excessive
sensitive and respond defensively to harmless substances. These conditions are
familiar to us as asthma and allergies -- conditions in which the body's defensive
systems have gone awry.

There is now evidence that early infection may protect against allergies in
later life. The researchers sometimes hypothesize this is because their immune
system is strengthened; a more coherent explanation is that their immune systems
are being calibrated correctly by this early experience. The current consensus
hypothesis is now that higher standards of hygiene deprive the developing immune
response of important signals during the period from the fetus to up to 5 years
after birth, signals important for immune system development. This hygiene hypothesis,
Holgate (1999) argues, "best accommodates the link between allergy and
social class, the urban to rural gradient, infant diet, over-use of antibiotics
and the East to West gradient of disease."

Martinez FD. Role of viral infections in the inception of asthma and allergies
during childhood: could they be protective? Thorax 1994; 49: 1189-91.

Summary

Background Infections in early childhood may prevent allergies
in later life. If this hypothesis is true, early exposure to childcare
outside the home would protect against atopy by promotion of cross infections.
We investigated whether children who attend a nursery at a young age have
a lower rate of atopy and fewer allergies than children who attend from
an older age.

Methods In a cross-sectional study carried out in 1992-93, we
examined 2471 children in three age-groups (5-7, 8-10, and 11-14 years)
from the towns of Bitterfeld, Hettstedt, and Zerbst in eastern Germany.
The children's parents answered a questionnaire about allergies and symptoms,
attendance at day care, and related factors. Sensitisation was assessed
by skin-prick tests and measurement of allergen-specific IgE antibodies
in serum.

Findings In 669 children from small families (up to three people),
the prevalance of atopy was higher among children who started to attend
day nursery at an older age than in those who started to attend at a younger
age (p<0·05). Compared with children who first attended at age
6-11 months, the adjusted odds ratios for a positive skin-prick test were
1·99 (95% CI 1·08-3·66) for children who attended
at age 12-23 months and 2·72 (1·37-5·40) for those
who attended at age 24 months and older. In 1761 children from large families
(more than three people), age of entry to day nursery had no effect on
atopy.

Interpretation Our findings accord with the hypothesis that early
infection may protect against allergies in later life.

One hypothesis about the origin of cancer is that it is an adaptation
of viruses to animal bodies. Animal bodies are designed to serve the purpose
of the whole organism; they are in a sense vast collaborative societies
of cells that have a common DNA and need each other to reproduce. Viruses
take advantage of the organization of the animal body to reproduce itself.
In this process, it may kill the animal, or it may simply absorb an increasing
amount of its resources to replicate itself..

The main source on this hypothesis is probably Nesse and Williams (1994);
Nesse may have written something more recent and more focused on the viral
theory of cancer. A specific instance of the hypothesis is that a "simian
virus known as SV40 has been associated with a number of rare human cancers.
This same virus contaminated the polio vaccine administered to 98 million
Americans from 1955 to 1963." It is presented in popular form in Bookchin
and Schumacher (2000).

Bibliography

Bookchin, Debbie and Jim Schumacher (2000). The Virus and the Vaccine.
Atlantic Monthly February 2000. Full
text (external).

Nesse, Randolph M. and George C. Williams (1994). Why We Get Sick: The New
Science of Darwinian Medicine. New York: Times Books.

Walbachia possible cause of
speciation in insects

Wolbachia infects as much as 20 percent of all insect species, and tends to
infect large percentages of the individuals in those species. It was first noticed
more than 50 years ago when scientists noticed that some species of mosquitoes
could interbreed when given antibiotics. In the 1970s researchers found that
a bacterium was responsible, and there has been speculation ever since about
whether the bacterium caused speciation of its host, or merely enhanced it.

Wolbachia propagates itself in an unusual manner: Instead of merely helping
its host compete against non-infected hosts as many parasites do, Wolbachia
actively seeks to eliminate non-infected hosts by stopping them from reproducing.
To do this, the parasite alters the sperm of its male host, rendering it infertile
when paired with an uninfected female. If, however, the male mates with an infected
female, the damaged reproductive cells are “rescued” by the female’s parasite.
It’s as if the bacterium encodes the sperm cell, rendering it useless unless
it encounters the de-coding bacterium from another infected wasp. The result
is that infected males can only impregnate other infected females, not uninfected
ones, and makes it difficult for uninfected females to find a compatible mate.
The trickery is part of a wider system that assures that as many wasps as possible
will pass Wolbachia on to the next generation. Infected males can have offspring
only with infected females, and infected females automatically pass the infection
on to all their offspring.

When attacked by herbivores, some plants are known to emit volatile compounds
that attract predators of the herbivores. However, these indirect defenses have
been demonstrated only in artificial laboratory and agricultural situations.
Kessler and Baldwin (p. 2141; see the Perspective by Sabelis et al.) show that
such systems also operate under natural conditions. Using plants of Nicotiana
attenuata growing in the Great Basin Desert in Utah, they directly manipulated
individual components of the suite of volatile organic compounds released after
herbivore attack by Manduca caterpillars. They identified compounds that dramatically
increased attack by the caterpillar's predators and that also reduced the oviposition
rate of the Manduca moth. Thus, the plant can exert both "bottom-up" and "top-down"
control of its enemies.

In Plagues and Peoples (1977), William H. McNeill points out that many
of the distinctive infectious diseases of human beings got their start in animals,
particularly domestic animals. Measles is probably related to canine distemper,
and influenza to a disease in hogs. For an elaboration of this theme in world
history, see also Jared Diamond's Guns, Germs, and Steel.